proccontrol: make -s require a target

Require a command to exec or a pid to target and update usage and the
manpage to make this more clear.

It makes no sense to invoke a procctl(2) command on the current process
only to exit. Users are sometimes confused about how proccontrol works
and think it effects their shell environment when invoked without a
target. Disallowing this nonsensical behavior and clarifiying usage
will hopefully reduce confusion.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D46422